Observation of the doubly charmed baryon ⁺cc

A search for the doubly charmed baryon ⁺cc in the ⁰cπ⁺ decay channel is performed using proton-proton collision data corresponding to an integrated luminosity of 6.3fb⁻¹, collected with the upgraded LHCb detector in 2024 at a center-of-mass energy of 13.6TeV. A peaking structure with a global significance of $8.7σ$ is observed in the ⁰cπ⁺ mass spectrum, where the ⁰c baryon is reconstructed in the pK⁻K⁻π⁺ final state. The structure is consistent with originating from a weakly decaying particle and is identified as the doubly charmed baryon ⁺cc. Its mass is determined to be 3725.9 ± 1.0 \,(stat) ± 0.2 \,(syst) ± 0.4 \,(lifetime) ± 0.6 \,(ext)\,MeV/c², where the third uncertainty arises from the dependence of the selection-induced bias on the unknown ⁺cc lifetime, and the fourth is due to the uncertainties on the masses of the ⁰c, ⁺c, and ⁺⁺cc baryons.